The Effects of Energy Balanced Metamorphic Reactions and Far-Field Tectonic and Gravitational Forces on Basin Formation

Hartz, E.H.^1,2, Podladchikov, Y.Y.¹, Medvedev, S.¹, and Faleide, J.I.^3
1Aker Exploration AS, Stavanger, Norway
²Physics of Geological Processes, University of Oslo, Oslo
³Department of Geosciences, University of Oslo, Oslo

In classical basin models, rift basins further develop into post-rift basins by thermal contraction of the uplifted astenosphere below the thinned lithosphere (thermal subsidence). These first generation models are still widely and often successfully applied, in part because the researcher can choose between symmetric (McKenzie, EPSL, 1978) or asymmetric (Wernicke, Canadian J. Earth Sci., 1985) lithospheric thinning, thereby displacing the focus of uplift and subsidence in space and time, in order to fit a given geological scenario. Despite their success, these models do not consider first-order petrological observations and some physical principles. For example, the density of rocks in these models varies only as a function of temperature. Classical petrology, however, demonstrates that rock density depends on both pressure and temperature. Furthermore, the mineral phase transitions that cause the most abrupt density changes are predominantly pressure sensitive.
What might be considered second generation of basin models was developed recently (e.g., Petrini et al., Terra Nova, 2001; Kaus et al., EPSL, 2005). These models account for mineral phase transitions and demonstrate that modeled subsidence differs up to one km when compared to results of earlier models. Our study takes a step further in exploring the effect of metamorphic reactions deep below basins. In what optimistically might be called a next generation of basin models, we add mass balanced metamorphic reactions, energy balanced hydration and dehydration reactions and melting, and force and energy balanced pressure and temperature calculations. To isolate the effects, we start with 1D models studying, for example, how compressed lithosphere may develop deep marine basins. In our modeled scenario eclogitization (c. 10 % volume reduction) of the gabbroic lower crust trigger affects that leads to formation of a 4 km deep basin. It is not trivial to differentiate eclogite (crust) from peridotite (mantle) by standard geophysical methods. Thus natural examples of the compressional model may appear like a classical rift basin formed above what appears like updomed Moho, but in reality is a metamorphic front within the crust. Such differences in interpretations have significant consequences for petroleum exploration. Most importantly, the basin produced in our new model is initially much colder, compared to classical rift models.
To complement our basin formation model and estimate tectonic stresses, we are developing a 3D mechanical model based on the extended thin sheet approximation (Medvedev & Podladchikov, Geophys. J. Int., 1999). Using this model we first compare the horizontal stress components of Earth gravitational disequilibrium’s to far field tectonic stresses. Then we debate how local topographic gradients may amplify far-field tectonic stresses, thereby possibly account for complex patchworks of subsidence and uplift. This effect, however, is possible only if the Earth lithosphere is modeled as a crumpled spherical shell. Models in which lithosphere is presented as flat plate or as part of a plain spherical shell do not display such effects. Finally we compare our models to natural examples on land the East Greenland margin, and in the Barents Sea.

AAPG Search and Discover Article #90066©2007 AAPG Hedberg Conference, The Hague, The Netherlands

AAPG Search and Discover Article #90066©2007 AAPG Hedberg Conference, The Hague, The Netherlands